As is well known in the art, primary oil recovery methods recover only about 15% of the oil in the reservoir, and classical secondary recovery methods, such as water flooding, extracts only about 30% of the oil. This means that more than 60% of the oil is entrapped in the holes and pores of the oil reservoir even after the employment of conventional EOR processes. This is due to the formation of a system called oil-wet in the reservoir, in which the remaining oil in the reservoir sticks to the reservoir rock as a thin layer and wets the rock. Therefore, as is well known to a person skilled in the art, to further increase oil extraction efficiency via separating the remaining oil from the rock surface, it is necessary to change the wettability of the reservoir from an oil-wet system to a hydrophilic or water-wet system.
Accordingly, the wettability of the reservoir can be modified using complementary methods, such as chemical enhanced oil recovery (hereinafter “C-EOR”). As is known in the art, in C-EOR methods, oil extraction efficiency is increased by injecting chemical compounds into the reservoir. The properties of these chemical compounds can be modified, such as by using techniques in the field of nanotechnology to create novel chemical compounds, which, in turn, can significantly improve the efficiency of C-EOR. In this method, the efficiency of the well can be increased by using nanofluids, nanoparticles, nano-surfactants, and nanocomposite hydrogels via the formation of a Pickering emulsion (an emulsion stabilized with solid nanoparticles) from the injected nanoparticles and the oil existing in the reservoir, facilitating the movement ability of the oil inside the reservoir, and decreasing the interfacial tension between the oil and water.
Different mechanisms are known in the art for the use of nanotechnology in EOR purposes, such as the use of polymeric nanocomposites, production and application of emulsions and nano-emulsions in enhanced oil recovery, the use of nano-surfactants, and injection of metallic nanomaterials into heavy oil reservoirs in order to increase thermal conductivity of the oil.
However, it should be understood that although many methods have been disclosed in the art on the application of nanoparticles in enhanced oil recovery, the efforts have so far led to only small improvements in the extraction of oil. The reason is that there are some parameters affecting the performance of the injected nanomaterials into the oil reservoirs which should be taken into consideration. As is well known in the art, nanoparticles do not disperse well in oil/water emulsions, and to overcome this issue, a method must be used to prolong the homogenous stability of nanoparticles in the oil/water emulsions.
The general use of carbon nanotube-based nanohybrids to increase the dispersion quality of the nanoparticles in the oil/water emulsions, as well as their stability, is shown in the prior art. However, CNT-based nanohybrids having both hydrophilic and hydrophobic sides, so that they can decrease the surface tension between the oil and the rocks, lead to a more desirable wettability in the reservoir, and consequently increase the oil recovery efficiency, are not shown in the art. However, it should be understood that the methods used to synthesize the aforementioned nanohybrids play an important role in the achievement of the aforementioned properties for the nanohybrids.
Functionalizing the carbon nanotubes, using a mixture of nitric acid and sulfuric acid to add hydrophilic properties to the nanohybrids, is also known in the art. However, this functionalizing method is difficult and time-consuming, and it is not a cost-effective method when used on larger scales in the oil and gas industry. Another drawback of the aforementioned method is that the prepared emulsion using as-produced nanohybrids is stable for only 10 days, and moreover, the water and oil contact angle is only changed about 10 to 20 degrees, which, as is known in the art, is not enough for a significant improvement in EOR efficiency. In addition, in some methods disclosed in the prior art, nanohybrid synthesis is carried out in a reactor, which is a costly method due to the need for complicated devices and instruments.
It should be understood, that the stability of the resultant emulsion is directly affected by the method used to embed nanoparticles into the CNT structure. For instance, coating CNTs on commercial silica nanoparticles is also shown in the prior art, resulting in an emulsion which is stable for a relatively short time.
It is, therefore, an object of the present invention to overcome the limitations, drawbacks and difficulties still existing in synthesizing, and employing nanohybrids in enhanced oil recovery applications.
It is also an object of the present invention to reduce the time and cost of the synthesis method of the aforesaid nanohybrids via using fewer additives and using inexpensive equipment.
It is a further object of the present invention to provide strong structures of CNTs-based nanohybrids, well dispersed in oil/water emulsions, which remain stable for a long time.
Also, an additional object of the present invention is to improve the EOR process using the aforementioned nanohybrids.
These and many other objects are met in various embodiments of the present invention, offering significant advantages over the known prior art and consequent benefits in the extraction techniques.